The ABR (Available Bit Rate) service in Asynchronous Transfer Mode (ATM) networks is intended to make the best use of remaining capacity after higher priority services such as CBR (Constant Bit Rate) and VBR (Variable Bit Rate) have been provided for. To achieve this, special ATM cells known as Resource Management or Rm cells are sent from the source, through the destination and back to the source to provide information on the congestion level in the switches in the path. This is used by the source to modify its transmission rate, the objective being to utilize link capacity fully while not losing any cells as a result of congestion. An Explicit Rate (ER) parameter is set in the Rm cells passing through the switch in the forwards and backwards directions, the ER being reduced as congestion occurs down to the Minimum Cell Rate (MCR) value to reduce the rate at which the source sends.
In addition, the Message Type field in the Rm cell includes Congestion Indication (CI) and No Increase (NI) bits which may be set respectively to signal to the source that a predetermined decrease in the transmission rate should be effected and that the source should not automatically increase the send rate--it is normally arranged to keep pushing the send rate up again after the ER has signalled a reduction, in order to ensure that the rate is not permanently depressed. The switch may generate a backwards Rm cell (known as BECN) to the source to indicate congestion, the rate of these being limited to 10 per second per connection.
The Explicit Rate or ER parameter thus dictates the maximum rate at which the source is allowed to send in cells per second. MCR dictates the minimum rate at which a source may always send. ER must therefore not be set less than MCR.